c++ implicit copy constructor and the assignment operator
I have a class defined as follows:
#include <iostream>
using namespace std;
class Point
{
int x, y;
public:
Point(int a, int b) : x(a), y(b)
{
std::cout << "Constructing point ( " << a << ", " << b << " ) "
<< std::endl;
}
Point(const Point& p) : x(p.x), y(p.y)
{
std::cout << "In copy constructor " << p.x << " " << p.y
开发者_StackOverflow中文版 << std::endl;
}
Point& operator=(const Point& p)
{
std::cout << "In assignment operator " << p.x << " " << p.y
<< std::endl;
x = p.x;
y = p.y;
return *this;
}
};
int main()
{
Point p1 = Point(1, 2);
return 0;
}
Now when I execute this all I see is Constructing point (1, 2)
. I am assuming that the compiler is doing some optimization here. Is it correct that in theory a temporary gets constructed and then the assignment operator is called to initialize p1?
No, in a declaration like that the =
operator actually still means to call a constructor, and the compiler probably elided away any possible copy construction as an optimization. =
in a declaration would never result in an assignment being called. So in theory a temporary could be created and copy-constructed into p1
.
If you want see the operator = called, you must write a code like this:
Point p1(5, 5);
Point p2(0, 0); //You don't have a default constructor.
p2 = p1; // Now operator = is called.
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